1. Field of the invention
The present invention relates to an ink which provides excellent color recording on plain paper such as wood-free paper, medium-quality paper, bond paper, and paper for copying. The present invention also relates to an ink-jet recording method, and an ink-jet recording apparatus employing the above ink.
2. Related Background Art
Personal computers including desk-top types, lap-top types, and book types have come to employ a color display unit in recent years. Software is also directing to color representation. Consequently, printers are changing from monocolor types to color types.
Among recording methods employed by such printers, ink-jet recording is attracting attention. In ink-jet recording, ink droplets are ejected and are attached to a recording medium, such as paper sheets, converted paper sheets, plastic films, and cloths without contact of a recording head to the recording medium. The ink-jet recording method is advantageous in that noise is not produced because of non-contact of the recording head with the recording medium, and high speed printing and color recording is practicable.
In the ink-Jet recording method, it is required that (1) ink does not cause running of ink at the recorded portion on the recording medium, (2) ink is sufficiently stable during storage, and (3) the safety is high. In color ink-jet recording, it is further required that (4) mixing(or bleeding) of different colors of inks caused by unfixed ink droplets does not occur on contact of different color inks on a recording medium, (5) a solid color portion is uniform in color, and (6) failure of ink ejection does not occur after interruption of ink ejection (first-ejection difficulty), since limited colors of inks are more frequently used in color printing, unlike monocolor printing.
However, conventional inks per se are not usable as inks for color recording because they cause serious bleeding and do not give high-quality images. Presumably, the bleeding is caused by the fact that the ink is dotted before the previously dotted different color of ink has been sufficiently fixed. Coated paper, which has high ink-absorbing ability, is used for color recording in order to prevent the bleeding. Further, for printing on plain paper such as paper for copying and bond paper, a printing method reflecting upon the fixing time of ink is proposed in which ink is dotted after the previously dotted ink droplet has been fixed. In such a printing method, the speed of the paper feed has to be slowed down disadvantageously, thereby the printing time becomes longer irrespectively of the ink-dotting method. Therefore, high speed printing, which is characteristic of ink-jet recording, cannot be practised with such a printing method.
To shorten the fixing time, Japanese Patent Application Laid-Open No. 55-29546 discloses a method in which a surfactant is incorporated into ink in a larger amount than usual and thereby the ink seems to become dry instantaneously owing to the increased penetration power of ink afforded by the surfactant. However, the simple addition of a surfactant in a larger amount as above cannot prevent mixing of different colors of inks at the border of colors even though it improves the ink fixation. Moreover, it augments penetration of ink toward the back face of the paper, which lowers color density on the paper face, or causes nonuniform dyeing of a fiber layer on the paper surface, resulting in irregularity of color density in solid printing area and deterioration of image quality.
Furthermore, feathering and ink fixation on a plain paper are improved by use of strongly alkaline ink (see Japanese Patent Application Laid-Open No. 56-57862, etc.), or by use of a specific kind of ink (see Japanese Patent Application Laid-Open No. 56-5871, etc.). These inks, however, do not always prevent bleeding satisfactorily, and even in the case where the bleeding is relatively slight, the feathering is conspicuous (not satisfying the aforementioned requirement (1) of the prior art) or ejection failure is liable to occur (not satisfying the aforementioned requirement (6) of the prior art). As above, no color ink has been obtained which satisfies all the above requirements and gives high quality of an image with high speed on plain paper.